The invention relates to a process for manufacturing objects of synthetic resin.
In FR-PS 1 366 921 a process is disclosed for the manufacture of thermoplastic parts made by injection molding with at least one opening to a hollow cavity. For this purpose the molten thermoplastic material is injected into a mold which consists of strong material of good thermal conduction, and in which at least one core is provided which consists of a hard material and is resistant to collapse and deformation during injection molding, but begins to melt at a temperature that is between the deformation temperature and the resoftening temperature of the thermoplastic material. After injection molding the shaped object with the core is heated to a temperature between the deformation temperature and the softening temperature of the shaped object so that the core will melt out without melting the thermoplastic material, in order thus to remove the core from the casting. An alloy of bismuth and tin is named as the material for the remeltable cores. After melting out, the metal is usable again and is supplied again to the casting process.
Furthermore, DE-OS 26 28 390 discloses a method and an apparatus for producing one or more hollow sand cores for casting molds. These so-called shell cores are made by mixing thermosetting binding agent with sand. This sand is shaken in a heated core box and after the core surface has hardened the entire core box is rotated 180.degree. and the excess sand that has not hardened is poured out. In this manner hollow cores are obtained which are more economical than solid cores due to the lesser amount of material and their quick preparation.
A disadvantage of the known state of the art is that, for one thing, a solid core requires the use of a large amount of material, but on the other hand the use of hollow cores, especially when metal is used as the core material, is complicated and at the same time there is no assurance that a uniform wall thickness will be maintained.